Technical Abstract:
Besides the physical defects due to the destructive Huanglongbing (HLB) citrus greening disease on oranges, the infected fruit and resulting juice have been perceived as being more sour, bitter and off-flavored. In the symptomatic juice, the off-flavor was correlated with lower sugars, and sometimes with higher acids. Nevertheless, differences in flavor between symptomatic and healthy fruit were also correlated with higher limonin levels, although they were below taste thresholds. It is important to mention that thresholds of limonin and nomilin have been previously reported in water but never in combination and or in a complex mixture. Therefore, as a preliminary step to understand and characterize what the metabolites are that are responsible for the bitter off-favor of HLB orange fruit, thresholds of limonin, nomilin, and their combination in a sugar and acid matrix, as well as in Valencia orange juice were determined by taste panels. Between 16 and 23 panelists a priori trained to differentiate bitter and sour tastes were asked to participate in the taste panels. Food grade limonin and nomilin were added alone or in combination to a simple matrix solution composed of 6% sucrose and 0.15% citric acid in drinking water; or to a complex matrix solution containing 5.2% sucrose, 2.1% glucose, 2.5 % fructose, 0.75% citric acid and 0.25% malic acid, or added directly into Valencia and commercial orange juice. All stimuli were prepared by dissolving the compounds in solution and once the highest concentrations were prepared, the rest of the samples were diluted in a series by a factor of 2. Each series of stimuli were presented 3 times on different days. A detection threshold was determined using a forced-choice ascending concentration series method of limits described in ASTM E679-91. At each selected concentration, a triangle sample set consisting of one test and two blank samples was presented to subjects; subjects were asked to determine which the test sample was. The subjects were also asked to rate the difference from 0 to 10. The best-estimate threshold for each panelist was considered to be the geometric mean of the concentration at which the last miss occurred and the next higher concentration. In the simple matrix, the threshold of limonin was lower than nomilin. Panelists perceived the nomilin bitterness as being more astringent, metallic and lingering than the limonin bitterness. The synergetic effect of limonin and nomilin was significant in decreasing their individual thresholds in a sugar and acid matrix. Interestingly, the thresholds of limonin and nomilin were lower in Valencia and commercial orange juice compared to the thresholds measured in the complex matrix. Our current results show that the threshold concentrations of limonin and nomilin in healthy ‘Valencia’ orange juice are higher than the concentrations of both limonin and nomilin found in HLB ‘Valencia’ fruit, which was perceived bitter by a taste panel. Perhaps the lower sugar and higher acid content of HLB fruit decrease the threshold of those bitter compounds.